In power systems, circuit breakers are used for connecting and disconnecting a load. During this process, the active elements of the circuit breaker either interrupt or incept high current, causing stresses in the circuit breaker as well as the connected power system components. The flow of the high current can be limited by closing and opening the circuit breaker at a specific instance on the source voltage waveform. A plurality of techniques are known for controlling the opening or closing of the circuit breaker in order to prevent generation of transient phenomenon. Such techniques rely on the usage of devices that perform synchronized switching control. One such device is the point on wave controller.
Point on wave controller is used for controlling switching instance of the circuit breaker. On receiving a command from a control unit, the point on wave controller advances the command to achieve closing or opening at an instance to minimize the current. The point on wave controller detects the opening or closing actuation time (also referred to as operating time) of the circuit breaker and calculates a time for switching in respect of the opening or closing command of the circuit breaker to ensure switching on a particular point on the voltage waveform. The point on wave controller determines the opening or closing actuation time as the time period between the instance at which the command was given to the circuit breaker and the instance at which electrical switching (i.e. interruption or inception of the electrical connection) happened.
Conventionally, in order to mitigate inrush currents, transformers are deenergised or opened at peak on the voltage waveform and energized or closed at the peak having the same polarity as the previous opening, on the voltage waveform. However, due to improper deenegisation and due to magnetic hysteresis, flux is often retained or left in transformer core.
When the effect of residual fluxes are not considered while deciding optimum targets for controlled energization of transformers, the resultant fluxes would be unsymmetrical and hence, heavy magnetic inrush currents will experienced while closing.
Conventionally, magnetizing inrush currents due to asymmetric flux have been mitigated by determining the residual flux in each phase based on load side voltage. The residual fluxes are calculated upon integration of transformer side voltage and hence, requires transformer winding side voltage measurements. In the absence such measurements due to unavailability of voltage transformers on transformer side, the residual fluxes cannot be directly estimated and hence effective mitigation of magnetizing inrush current is not possible.
This is further exacerbated in cases where there is interphase coupling between the phases of the transformer. In such cases, magnetic fluxes, which is to be estimated in each phase, would be dependent on the residual flux remaining on that phase and the flux induced by an interconnected phase.
Therefore, in light of the abovementioned discussion, there is a need for a method and system that satisfies the above mentioned problems.